FIG. 2 represents a thick-film hybrid circuit structure 10 in which a conductor 18 is formed on an alumina substrate 12, and a circuit component 14 is electrically and mechanically connected to the conductor 18 with solder 16. High-conductivity thick-film conductors of the type represented in FIG. 2 are typically formed from a paste containing a metal powder composed of silver, silver and palladium, a silver-palladium alloy, or admixtures of other metal powders. In addition, the paste typically contains glass and/or metal oxide particles as inorganic binders, and an organic vehicle system that serves as a binder for the paste. The conductor paste is deposited on the alumina substrate 12 and fired, such that the organic binder is removed and the powder particles and inorganic binders fuse to form the solid conductor 18. The solder 16 used to secure a circuit component 14 to the conductor 18 is typically a tin-lead composition, frequently near the eutectic composition, with relatively high tin content to allow for lower processing temperatures. FIG. 1 represents a cross-sectional view through a prior art conductor 20, which is depicted as being composed of metal particles 22 in direct contact with each other and the solder 16 (for simplicity and clarity, inorganic binder particles are not shown).
The operability of the hybrid circuit 10 depicted in FIG. 2 requires that the bond between the conductor 18 and solder 16 survive numerous thermal cycles, necessitating that the conductor 18 exhibit adequate solderability such that solder initially wets the conductor surface, and that adhesion between the conductor 18 and solder 16 is suitably maintained over an extended period of time. When hybrid circuits are used in high temperature applications over extended periods, interactions that occur between the conductor 18 and solder 16 can ultimately lead to degradation of the mechanical integrity of the assembly. It is generally recognized that when a silver-based conductor is soldered with a tin-lead solder, a brittle intermetallic region is formed between the conductor 18 and solder 16 during soldering, and may subsequently increase in thickness during high temperature exposure. The intermetallic region is generally the weak mechanical link of the conductor-solder bond, and is particularly prone to fracturing during thermal cycling due to the differing coefficients of thermal expansion of the intermetallic and the conductor and solder materials, which induces significant stresses at the interface between these structures. Solders, such as tin-lead solders, that exhibit high solderability on thick film conductors tend to promote the formation of this intermetallic region at the conductor-solder interface with metallic conductors. Such solders generally have a greater tendency to leach elemental metal from the conductor 18 during soldering, and promote diffusion of tin from the solder 16 into the conductor 18 during high-temperature exposure. In many applications, the growth of the intermetallic region is limited by the relatively low service temperature of the circuit 10. However, where relatively high temperatures are encountered over long periods of time, the size of the intermetallic region increases significantly, which generally exacerbates the detrimental effect that the intermetallic region has on the service life of the circuit structure.
Accordingly, it would be desirable if the leach resistance of a metallic thick-film conductor could be increased, and its resistance to diffusion of tin increased, so as to minimize the size of the intermetallic region that forms between the conductor and tin-lead solders at elevated temperatures. In doing so, it would be necessary to maintain the wettability and electrical conductivity of the conductor. However, approaches to date to improve leach resistance and tin-diffusion resistance have included increasing the palladium content and the glass content of the films, both of which have had the undesirable effect of increasing the resistance of the conductor.